Such systems are described, for example, in French patent specifications Nos. 2 389 865 and 2 441 145 which additionally provides for simultaneous guidance systems controlling a plurality of projectiles fired at one or more targets which are tracked and displayed.
It is desirable to protect the tracking of the projectile(s) from interference due to countermeasures which an enemy may attempt to deflect the projectile from its trajectory. Since the projectile appears on the screen as a bright spot due to radiation emitted by a projectile tracer, countermeasures consist in ejecting decoys close to the direction taken by the projectile, said decoys emitting similar bright spots, e.g. by means of radiation produced by pyrotechnical means. The aim is to create confusion in the tracking system between the bright spot corresponding to the projectile and the bright spot corresponding to the decoy. Such confusion ends up with the tracking system latching onto the decoy and tracking the decoy rather than the bright spot corresponding to the projectile.
The probability of success when using such countermeasures depends on the number of decoys used, their brightness (since tracking systems generally tend to latch onto the brightest spot) and above all on the accuracy of firing. The probability of countermeasure success is very high if the trajectory of the decoy crosses the trajectory of the projectile as seen in the video image. In such a case, there comes a moment when the two bright spots are superposed, subsequently, when they separate the tracking system has no way of recognizing which of the two bright spots corresponds to the decoy and which corresponds to the projectile. If, as suggested above, the decoy is brighter than the projectile (and pyrotechnical means are readily available for creating very bright sources of radiation) the tracking system is very likely to latch onto the decoy. It then becomes impossible to follow the projectile unless the operator is able to intervene manually, and there is usually not enough time for such intervention to take place. The present invention seeks to reduce the risk of the tracking system being confused by such decoys, by enabling the system to continue tracking the bright spot which corresponds to the projectile in spite of other bright spots constituted by the decoys.
For reasons of clarity in explanation, the following description concerns protecting a single projectile from a single decoy. However, the method is naturally applicable to projecting a projectile from a plurality of decoys fired simultaneously by multiplying each of the decoy steps of the method by the number of decoys. Similarly, the system may track a plurality of projectiles simultaneously as in the systems described by the above-mentioned prior documents. Finally, bright spot tracking may be protected against interference from decoys when the tracked bright spot represents an object other than a projectile, for example when it represents a target which emits radiation either directly (an aircraft jet) or indirectly (using illumination from a laser).